This invention relates to polymeric pigment dispersants prepared by reacting an isocyanate with a hydrophilic poly(ethylene glycol)alkyl ether, a hydrophobic polyester or polyacrylate, and a diamine.
The materials described in this application are cationic dispersants. The cationic substitution allows their use with anionic pigments, particularly carbon black and surface treated organic pigments. In addition, the use of mixed arms provides broader solubility characteristics and utility with a wider range of coatings vehicles. These materials can therefore be considered xe2x80x9cuniversal dispersantsxe2x80x9d.
There are currently several trends in the coatings industry which dictate the need to use cationic dispersants, and are therefore relevant to this invention. First, the use of isocyanate crosslinked coatings is becoming more important because, in part, they generally do not have a cure inhibition problem with cationic dispersants. Also, partially condensed melamine resins show increasing importance as used in coatings, because they are generally not so strongly inhibited by cationic dispersants. Yet another trend is that pigment suppliers are providing more pigments, particularly phthalocyanines, which have anionic surface treatments (i.e.; the pigment is acidic) which made cationic dispersants the ideal choice to achieve strong pigment-to-dispersant interaction.
The lack of emphasis in prior work on cationic functional dispersants, particularly those comprising tertiary amines, was at least partly caused by the use of dispersants in industrial and automotive coatings, which were formulated to depend on acid catalysis of melamine resins for crosslinking. Amine functional dispersants usually cause some cure inhibition in such coatings. The urea functionality incorporated using ammonia was sufficiently adsorbed on many pigments and did not interfere with the cure mechanism.
The present invention differs from prior art dispersants in that it encompasses a unique combination of hydrophobic arm(s), hydrophilic arm(s) and amine arm(s) functionality on an urethane backbone.
The present invention provides a pigment dispersant comprising a urethane polymer having at least three urethane groups, wherein
(a) a hydrophilic poly(ethylene glycol)alkyl ether;
(b) a hydrophobic polymer selected from the group consisting of polyesters and polyacrylates; and
(c) a diamine comprising a primary amine group and a tertiary amine group.
The isocyanate oligomer may be chosen from isocyanurates or biurets of toluene diisocyanate(xe2x80x9cTDIxe2x80x9d), hexamethylene diisocyanate (xe2x80x9cHDIxe2x80x9d), and mixtures thereof.
The dispersants described herein are the reaction product of a multifunctional isocyanate with a polyester or polyacrylate, a polyethylene ether and a diamine. The polyester or polyacrylate and polyethylene ether xe2x80x9carmsxe2x80x9d have widely different solubility profiles and provide entropic stabilization for the dispersant in a wide range of solvent systems. The amine xe2x80x9carmxe2x80x9d provides a functional group that can strongly interact with an acidic pigment for dispersion stability. Most preferably, the dispersants of this invention comprise the reaction product described above wherein 10 to 50 mole % of the isocyanate groups on the oligomer are reacted with a polyester or polyacrylate, 3 to 30 mole % are reacted with the poly(ethylene glycol) methyl ether and 30 to 65 mole % of the isocyanate groups are reacted with the diamine, with the proviso that 100% of the isocyanate groups are reacted.
Isocyanurates that may be used in this invention to provide the isocyanate oligomer are obtained from toluene diisocyanate (xe2x80x9cTDIxe2x80x9d; Desmodur IL), hexamethylene diisocyanate (xe2x80x9cHDIxe2x80x9d; Desmodur 3300); mixtures of TDI and HDI (Desmodur HL). A biuret that may be used is that obtained from hexamethylene diisocyanate (Desmodur N). These may be obtained from sources indicated in the Examples.
The isocyanate oligomers must have an average functionality of three or more, meaning that a molecule of the isocyanate contains, on statistical average, at least three free isocyanate groups. The average functionality, as opposed to the absolute number of isocyanate groups, is used because the isocyanates are obtained as isomeric mixtures of isocyanates having 3, 4, 5 or more functional groups. The average functionality can be determined experimentally by titrating to determine the weight % of isocyanate, determining the number average molecular weight (xe2x80x9cMnxe2x80x9d) of the oligomer (such as by Gel Permeation Chromatography xe2x80x9cGPCxe2x80x9d), and then calculating the average number of isocyanate groups.
The dispersants of this invention are prepared by sequentially reacting the polyester or polyacrylate, the poly(ethylene glycol), and the diamine with the selected isocyanate oligomer. At the conclusion of the synthesis, all of the isocyanate groups have been reacted.
The polyesters suitable for use in the invention have the general formula:
HO(C5H10xe2x80x94CO2)nR
where R is an alkyl of 1 to 12 carbons and n is an integer of 6 to 10. These polyesters are conveniently prepared by reacting caprolactone with an alcohol. The length of the polyester arm for any particular dispersion is determined by a balance between its being long enough to give good pigment dispersion and yet not so long that it crystallizes too readily to produce a unstable solution. A typical synthesis of the polyester arm using n-decanol and caprolactone, is shown in the following figure: 
The length of the arm containing six repeat units (i.e., n=6) is calculated at 6.63 nm and the length of the arm containing nine repeat units (n=9) is 9.25 nm.
Polyacrylates useful in the present invention are hydroxy-terminated (meth)acrylic polymers. Such polymers are prepared by reacting (meth)acrylic monomers with 2-mercaptoethanol in the presence of a free radical initiator. Azo initiators such as azobisisobutyronirile (VAZO(copyright) 64, E. I du Pont de Nemours and Co., Wilmington, Del.) are particularly useful. Other useful monomer compositions and initiating conditions are described in U.S. Pat. No. 4,032,698, the disclosure of which incorporated herein in its entirety. The preparation of the polyacrylate arm is illustrated schematically by the following equations: 
Poly(ethylene glycol)alkyl ether polymers useful in this invention are those having the general formula
H(Oxe2x80x94CH2xe2x80x94CH2)nxe2x80x94OR
where n is an integer of 15 to 67 and R is an alkyl of 1 to 4 carbons, preferably methyl. The poly(ethylene glycol)alkyl ethers have a number average molecular weight (xe2x80x9cMnxe2x80x9d) between 750 and 3000, preferably between 900 and 2500, and all ranges encompassed therein. The poly(ethylene glycol)alkyl ethers have a single hydroxyl functional group. Such polymers are commercially available from Aldrich Chemical and other sources. Alternatively, poly(ethylene glycol)alkyl ether polymers can easily be prepared using conventional techniques well known to those skilled in the art.
Suitable diamines useful in the invention are those having a primary amine and a tertiary amine. Such diamines have the general formula: 
where n is an integer of 2 to 5, preferably 3, R1 and R2 are each independently an alkyl of 1 to 4 carbons or together form a saturated or unsaturated 5 to 8 member ring optionally containing N or O. Three diamines which are preferred are: 
1-(3-aminopropyl)imidazole is the most preferred. It has the complexing activity of pyridine, but is more basic. It has a relatively low equivalent weight, and is commercially available. In the experimental results reported herein, it was necessary to use a 10 to 15% calculated excess of isocyanate to ensure complete consumption of all of the diamine used in the last step of the synthesis.
The dispersants of this invention can thus be schematically illustrated by the figure below, in which a tri-functional isocyanate has been reacted with a diamine, a polyester and a poly(ethylene glycol)methyl ether. It should be recognized that, in accordance with the invention, the polyester could be substituted with a polyacrylate in the figure below. 
The dispersants of this invention are useful in making pigment dispersions and mill bases for paints and other coatings. To form a pigment dispersion or a mill base, pigments are added to the dispersant and the pigments are dispersed using conventional techniques such as high speed mixing, ball milling, sand grinding, attritor grinding or two or three roll milling. The resulting pigment dispersion has a pigment to dispersant binder weight ratio of 100/1 to 100/500.
Any of the conventional pigments used in paints can be used to form the pigment dispersion such as metallic oxides like titanium dioxide, iron oxides of various colors, zinc oxide, carbon black, filler pigments such as talc, china clay, barytes, carbonates, silicates and a wide variety of organic pigments such as quinacridones, phthalocyanines, perylenes, azo pigments, indanthrones, carbazoles such as carbazole violet, isoindolinones, thioindigo reds, benzimidazolinones, metallic flakes such as aluminum flake, pearlescent flakes and the like.
It may be desirable to add other optional ingredients to the pigment dispersion such as antioxidants, flow control agents, rheology control agents such as fumed silica, microgels, UV stabilizers, screeners, quenchers and absorbers. The pigment dispersions can be added to a variety of solvent borne or aqueous coating compositions such a primers, primer surfacers, topcoats which may be monocoats or basecoats of a clear coat/base coat finish. These compositions preferably have an acrylic polymer or polyester polymer or a blend of these types of coating vehicle as the film forming constituent and may also contain crosslinking agents such as blocked isocyanates, isocyanates, alkylated melamines, epoxy resins and the like. Other film forming polymers can also be used, such as acrylourethanes, polyester urethanes, polyethers and polyether urethanes that are compatible with the pigment dispersion.
The following examples illustrate the invention. All parts and percentages are on a weight basis unless otherwise indicated. Molecular weights are determined by gel permeation chromatography using polystyrene as the standard and tetrahydrofuran as the carrier solvent.